The increasing data rates require the equipment used in transmission reception, as well as the medium, to conform to the various standards to maintain accuracy of signals. Accordingly test and measurement instruments need to be able to make such measurements, taking into account the increased electromagnetic radiation effects.
In physics, the signal is usually a wave, such as an electromagnetic wave, random vibration, or an acoustic wave. Power spectral density (PSD) of the signal is the spectral density of the wave multiplied by an appropriate factor. PSD is the power carried by the wave. Power spectral density is commonly expressed in watts per hertz (W/Hz) or dBm/Hz.
The signals in 10GBASE-T are PAM (Pulse Amplitude Modulation) level with random patterns. These signals are sent over four lanes from the Transmitter on Cat 5 or Cat 6 cables. By measuring PSD and power level, we can ensure that the external interference and adjacent channel interference are sufficiently low such that it does not alter the signal level at the far end so that its encoded digital data can be decoded without any error.
Engineers who use oscilloscopes in their work commonly do not think that an oscilloscope is capable of making the PSD measurement required by 10GBaseT applications. They commonly associate the 8-bit A/D converter in an oscilloscope as only being able to provide a 48 dB dynamic range, and they normally think the −117 dBm/Hz mask level can only be measured by a spectrum analyzer.
It is known in the art that 10GBASE-T Ethernet technology has emerged from 1000 BASE-T to operate at the 10 Gb/sec speed over the same CAT 5 or CAT 6 network cables. This is accomplished by providing four lane transmission, by the use of more efficient smart codes, and by employing DSP technology. The main application of 10G is the gigabit switch uplink which is used for server clustering, data center interconnects, and it may also take part in desktop computer in future.
10 Gb/sec speed is accomplished by transmitting 2.5 Gb differential signals over a 4-pair cable with each pair aggregating to 10G. When the signaling uses 16 PAM levels, it is important that interference should be kept to minimal. Otherwise, the interference alters the signal level and subsequently error occurs on the transmission. The data is sent over the Cat 5 or Cat 6 cable. Channel limits play a vital role in the near-end cross talk, far-end cross talk, return-loss. So, DSP has been designed to provide the required suppression up to 150 dB/Hz.
10GBASE-T uses a two-dimensional code and is created using a pair of adjacent PAM 16 symbols. The distance between the adjacent points is increased using DSQ 128 constellation to provide immunity to noise. Each pair of wire operates at 800 MHz symbol rate, which puts Nyquist frequency for baseband signaling at 400 MHz. So the upper frequency limit of 500 MHz for the cable will be good enough to carry the signal.
Power spectral density is a method of scaling the amplitude axis in certain spectral values which consist of random signals rather than deterministic signals. Because a random signal has energy spread out over a frequency band, it is not meaningful to speak of its RMS value at any specific frequency. It makes sense to consider its amplitude in a fixed frequency band, usually 1 Hz. PSD is defined in terms of amplitude squared per hertz and is thus proportional to the power delivered by the signal in a one-hertz band.
It is important that a received signal be immune to possible noise interference when we run the cable with pairs in close proximity to each other. The source of the interference is ANEXT (Alien Near end cross talk, Alien Far end cross talk). The typical power spectral density of that interference is shown in prior art FIG. 1.
Referring to prior art FIG. 1, a graph of Signal and Noise Spectra (Ptx=4.2 dBm, L=11 m) includes a number of plots shown as a function of PSD vs. frequency. Specifically, plot 110 is a plot of the received signal (Rx); plot 112 is a plot of the total noise; plot 114 is a plot of −48 db txD Floor; plot 116 is a plot of 10G ANEXT; plot 118 is a plot of 10G AFEXT; plot 120 is a plot of 1G ANEXT; plot 122 is a plot of −147 Rx Noise; plot 124 is a plot of 9.0b ADC noise; and plot 126 is a plot of −150 Background noise.
For Interoperability, it is necessary to ensure the SNR margins are good enough in the presence of the 1000 BASE-T and background noise. Maximum output power is necessary to manage the levels of 10GBASE-T ANEXT and AFXT. So, the mask for the PSD is defined such that the upper mask provides an EMI-based bound for the signal and lower mask ensures that the output stream is compatible with expected equalizer capabilities.
The plot of power spectral density Vs frequency is shown in prior art FIG. 2, in which line 210 is a mask upper limit line as specified in IEEE Ethernet standards document, plots, 212 and 214 are PSD spectral plots of existing Ethernet signals. Plot 216 is the mask lower limit line. The plot of power spectral density Vs frequency of FIG. 2 may be transformed to the PSD limit as:
Upper Limit
      Upper    ⁢                  ⁢    PSD    ⁢                  ⁢          (      f      )        ≤      {                                                      -              78.5                        ⁢                                                  ⁢            dBm            ⁢                          /                        ⁢            Hz                                                0            <            f            ≤            70                                                                          -              78.5                        -                                          (                                                      f                    -                    70                                    80                                )                            ⁢                                                          ⁢              dBm              ⁢                              /                            ⁢              Hz                                                            70            <            f            ≤            150                                                                          -              79.5                        -                                          (                                                      f                    -                    150                                    58                                )                            ⁢                                                          ⁢              dBm              ⁢                              /                            ⁢              Hz                                                            150            <            f            ≤            730                                                                          -              79.5                        -                                          (                                                      f                    -                    330                                    40                                )                            ⁢                                                          ⁢              dBm              ⁢                              /                            ⁢              Hz                                                            730            <            f            ≤            1790                                                                          -              116                        ⁢                                                  ⁢            dBm            ⁢                          /                        ⁢            Hz                                                1790            <            f            ≤            3000                              
Lower Limit
      Lower    ⁢                  ⁢    PSD    ⁢                  ⁢          (      f      )        ≥      {                                                      -              83                        ⁢                                                  ⁢            dBm            ⁢                          /                        ⁢            Hz                                                5            ≤            f            ≤            50                                                                          -              83                        -                                          (                                                      f                    -                    50                                    50                                )                            ⁢                                                          ⁢              dBm              ⁢                              /                            ⁢              Hz                                                            50            <            f            ≤            200                                                                          -              86                        -                                          (                                                      f                    -                    200                                    25                                )                            ⁢                                                          ⁢              dBm              ⁢                              /                            ⁢              Hz                                                            200            <            f            ≤            400                              
Power spectral density (PSD) refers to the amount of power per unit (density) of frequency (spectral) as a function of the frequency. The PSD describes how the power (or variance) of a time series is distributed with frequency. By knowing the power spectral density and system bandwidth, the total power can be calculated.
To qualify the 10G BASE-T, we have to measure the power spectral density of the transmitter signal and ensure that spectral density is within the upper limit and lower limit of the mask. We also need to measure the power level such that it meets the requirement.
At present, the PSD is measured using a spectrum analyzer and noise marker function. Here, we have to move the noise marker based on the required frequency resolution, then log the results and subsequently plot the PSD curve, and then check for limit violations.